1. Field of the Invention
This invention relates, in general, to laser printers and, more specifically, to registration enhancing systems for hard copy scanning printers, including laser printers.
2. Description of the Prior Art
In some applications, the location of the printed image upon the hard copy or sheet material on which the printed image is produced is not critical to acceptable operation of the printer. Generally speaking, precise printers, such as laser printers, which are used to produce a single copy of an image, perform satisfactorily if the image is registered on the hard copy paper within a fraction of an inch of the true registration position. However, with more and more printers using images stored in digital memory to construct the image on the hard copy material, more precise control of the location of the image is possible than with optical type duplicators and copiers.
Registration is particularly important when the printer is being used in some fashion for color printing or reproduction. In many color printing systems, several colors are printed separately on top of each other to form a composite color image. In order to have a good quality color image, it is necessary that the individual colors be aligned or registered very accurately on the printed paper. Color registration which is off just a very small fraction of an inch will produce color images that are not acceptable to present standards. Precise registration is also desired for machines which make masters to be used in color printing operations. With the resolution of such machines being greater, in some cases, than 1,000 lines per inch, the opportunity exists for producing very sharp composite color images if the registration between the different color masters is properly maintained.
Lack of registration in laser printers occurs for a variety of reasons. One of the reasons involves the synchronization of the laser scanning beam with the surface onto which the beam is focused. One type of currently available laser printer uses a photoconductive web material constructed in an endless belt configuration which rotates around rollers to move the latent image created by the laser beam through the other stations of the printer to eventually produce the hard copy output. With this type of printing system, the scanning of the laser beam must be coordinated with the speed and position of the web and with the electronic control of the modulation, or illumination control, of the laser beam. Typically, such systems include a means for scanning the laser beam linearly across the web surface to form one line of the image at a time. Successive line scans are used to construct an entire image on the web. Normally, the web is moving to provide a displacement from line to line, and the laser beam moves from one end of the line to the other end of the line.
Many printers operate with the location of the web and the location of the laser beam on the scan line not always synchronized to each other. In other words, when the web is in a position to receive the image to be constructed by the laser beam, the laser beam may be already oriented to scan somewhere within the line. Since the scan is always started at the beginning of the line, a delay is necessary before the scan is commenced. The amount of delay depends upon the position of the laser beam within the line when the image is to be started. Starting of the image does not occur until the laser beam returns to the edge of the web and trips or is detected by a photodetector which signifies that the laser beam is in a position to start accepting image information which will be written or imaged onto the photoconductive web. As a result, it is possible, in a worst case analysis, that the laser beam would just have passed the edge detector when it is to start writing an image on the web. Since the image does not start writing until the laser beam scans the entire line and starts again at the edge, significant misregistration on the web can occur. In this case, the image written onto the web can be misregistered by an amount equal to the width of one scan line. When the printer is used to make masters for multicolor printing processes and to make color prints, this type of misalignment in the registration between the various color masters or the print layers produces undesirable copies, or prints.
Therefore, it is desirable, and it is an object of this invention, to provide a laser printer which produces hard copy images which are registered at an exact location more accurately than with laser printers constructed according to the prior art.